Background images are images that you display underneath your data in order to add more context to the marks in the view. A common use of background images is adding custom map images that correspond to a coordinate system in your data. For example, you may have data that corresponds to several floors in a building. You can use background images to overlay that data on the actual floor plan of the building to give more context. Other examples of using background images include showing a model of the sea floor, images of web pages for analyzing web logs, and even levels from video games to visualize player statistics.
While Tableau allows you to load dynamic maps from the online and offline provider, background images allow you to use your own custom images whether they are special maps or any other image that corresponds to your data.
Adding Background Images
When you add a background image to the view, you need to specify a coordinate system by mapping both the x and y axes to the values of fields in your database. If you are adding a map, the x and y axes should be longitude and latitude expressed as a decimal. However, you can map the axes to any relevant fields based on your own coordinate system.
To add a background image:
1. Select Map > Background Images and then select a data source.
2. In the Background Images dialog box, click Add Image.
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3. In the Add Background Image dialog box do the following:
Type a name for the image into the Name text box.
Click Browse to navigate to and select the image you want to add to the background. You can also type a URL to link to an image hosted online.
Select the field to map to the x-axis of the image and specify the left and right values. When adding a map, the longitude values should be mapped to the x-axis using decimal values (instead of degrees/minutes/seconds or N/S/E/W).
Select the field to map to the y-axis of the image and specify the top and bottom values. When adding a map, the latitude values should be mapped to the y-axis using decimal values (instead of degrees/minutes/seconds or N/S/E/W).
You can adjust the intensity of the image using the Washout slider. The farther the slider moves to the right, the more faded the image will appear behind your data.
4. You can specify the following options using the Options tab:
Lock Aspect Ratio – select this option to maintain the original dimensions of the image for any manipulations of the axes. Deselecting this option allows the image’s shape to be distorted.
Always Show Entire Image – select this option to avoid cropping the image when the data encompasses only a portion of the image. If you lock both the axis in a view, this option may be negated.
Add conditions for when to show the image. Refer to Adding Conditions to Filters to learn more about defining conditions.
5. Click OK.
When you add the x and y fields to the Rows and Columns shelf in the view, the background image displays behind the data. If the background image does not display, make sure that you are using the dis-aggregated measures for the x and y fields. You can dis-aggregate all measures by selecting Analysis > Aggregate Measures or change each measure individually by right-clicking the field on the shelf and selecting Dimension.
Finally, if you’ve used the generated Latitude and Longitude fields for the x and y fields, you’ll need to disable the built in maps before your background image will display. Select Map > Background Maps > None to disable the built-in maps.
In order to make the marks in a view more visible when placed on top of a background image, each mark is surrounded by a solid contrasting color called a halo. You can turn mark halos off by selecting Format > Show Mark Halos.
Setting up the View
After you add a background image, you need to build the view in a way that matches the x and y mappings you specified for the image. That is, the fields you specified as x and y must be on the proper shelves.
Follow the steps below to set up the view correctly:
Place the field mapped to the x-axis on the Columns shelf.
If you are working with maps, the longitude field should be on the columns shelf. It may seem backward at first, however, the fields on the columns shelf determine the values distributed across the x-axis.
Place the field mapped to the y-axis on the Rows shelf.
If you are working with maps, the latitude field should be on the rows shelf. It may seem backward at first, however, the fields on the rows shelf determine the values distributed across the y-axis.
You can add several background images to the workbook and then select the image or images to make active on each sheet. The Background Images dialog box lists all of the images, the required fields, and whether they are visible. The visibility is determined based on whether the required fields are used in the current view.
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